Tap & Drill Chart
SAE UNC/UNF & Metric · Drill bit sizes
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Unlock Full Bible — $9.95Tap & Drill Size Chart — SAE & Metric
Find the correct drill bit size for any tap. SAE UNC/UNF and metric thread tap drill sizes in fractional, letter, and mm. Includes diesel sensor tap sizes (M18 NOx, M6 sensor).
SAE Tap Drill Sizes
75% thread engagement (standard)
| Tap Size | Drill Bit | mm |
|---|---|---|
| 1/4"-20 UNC | #7 (0.201") | 5.1 mm |
| 1/4"-28 UNF | #3 (0.213") | 5.4 mm |
| 5/16"-18 UNC | F (0.257") | 6.5 mm |
| 5/16"-24 UNF | I (0.272") | 6.9 mm |
| 3/8"-16 UNC | 5/16" (0.3125") | 7.9 mm |
| 3/8"-24 UNF | Q (0.332") | 8.4 mm |
| 7/16"-14 UNC | U (0.368") | 9.3 mm |
| 1/2"-13 UNC | 27/64" (0.4219") | 10.7 mm |
| 1/2"-20 UNF | 29/64" (0.4531") | 11.5 mm |
| 5/8"-11 UNC | 17/32" (0.5313") | 13.5 mm |
| 3/4"-10 UNC | 21/32" (0.6563") | 16.7 mm |
Metric Tap Drill Sizes
75% thread engagement (standard)
| Tap Size | Drill (mm) | Fractional | Note |
|---|---|---|---|
| M5 × 0.8 | 4.2 mm | 11/64" | |
| M6 × 1.0 | 5.0 mm | 13/64" | Sensor threads |
| M7 × 1.0 | 6.0 mm | 15/64" | |
| M8 × 1.25 | 6.8 mm | 17/64" | |
| M8 × 1.0 | 7.0 mm | 9/32" | Fine thread |
| M10 × 1.5 | 8.5 mm | 21/64" | Very common |
| M10 × 1.25 | 8.75 mm | 11/32" | Fine thread |
| M12 × 1.75 | 10.2 mm | 13/32" | |
| M12 × 1.25 | 10.8 mm | 27/64" | Fine thread |
| M14 × 2.0 | 12.0 mm | 15/32" | |
| M16 × 2.0 | 14.0 mm | 35/64" | |
| M18 × 2.5 | 15.5 mm | 39/64" | O2/NOx sensors |
Pro tip: Always drill 75% thread depth for diesel sensor ports — 100% thread engagement causes galling and seized sensors.
Need diesel sensor thread repair procedures?
The full Bible includes complete thread repair procedures for diesel sensor ports — NOx, O2, boost, fuel rail, DEF, and EGR sensor threads — with Helicoil and Time-Sert insert specifications by engine family.
Unlock the Full Bible — $9.95Tap & Drill — FAQ
What drill bit do I use for a 3/8"-16 tap?
A 3/8"-16 UNC tap requires a 5/16" (0.3125") drill bit, which is approximately 7.9 mm. This gives approximately 75% thread engagement, which is the standard for most applications.
What drill bit do I use for an M10 × 1.5 tap?
An M10 × 1.5 tap requires an 8.5 mm drill bit (approximately 21/64"). This is one of the most common tap sizes on diesel engines.
What drill bit do I use to extract a broken bolt before tapping?
Use a left-hand drill bit slightly smaller than the bolt diameter. Left-hand bits often remove broken bolts without needing an extractor. If the bolt does not back out, drill to the tap drill size for the original thread, then use a Helicoil or Time-Sert thread repair insert.
What drill bit do I use for an M18 × 1.5 NOx sensor tap?
An M18 × 1.5 fine-thread tap (used for NOx and O2 sensors) requires a 16.5 mm drill bit. This is a fine-thread tap — do not confuse with M18 × 2.5 coarse thread which requires a 15.5 mm bit.
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